Precision of copying machines has been remarkably improved in recent years, and popularization of electrophotographic color copying machines, in particular, has made it easy to falsify securities and so forth.
To prevent such falsification, various falsification prevention means have been proposed, and one of them utilizes the property of existing copying machines in that they cannot reproduce an original having brightness, such as metallic colors and interference colors.
Japanese Utility Model Laid-Open No. 168754/1988, for example, proposes paper which disposes a bright plate such as an aluminum foil having remarkable metallic colors on the upper surface of a substrate sheet and puts characters and patterns on the surface of the bright plate, and which cannot be reproduced on a copying machine. When this paper is copied, the foil surface becomes dark upon irradiation of light by the copying machine, so that the characters and patterns on the sheet surface cannot be read.
This type of paper has the advantage that reproduction itself is not possible (the resulting copy cannot be read), but is not free from the problem that because the occupying area of the bright plate such as the aluminum foil in the sheet of paper is great, the metallic luster colors are excessively stressed and provide an offensive feel. Further, the production process of paper becomes complicated and the cost of production becomes inevitably higher. Another problem lies in that recovery of paper-making fibers from waste or used paper becomes difficult.
The inventors of the present invention have conducted intensive studies so as to solve these problems and have come to realize that the original and the copy can be distinguished from each other if paper provides a different hue from that of the original even reproduction is made on a color copying machine.
Therefore, the present inventors have first examined a method which fragments a silver aluminum-metalized polyester film having high brightness into thin fragments and incorporates them into paper. When the resulting sheet of paper is reproduced on the color copying machine, the portions where the thin fragments are mixed are merely reproduced in black because the metallic luster cannot be reproduced. Accordingly, the original and the copy can be distinguished, and this type of paper is found to have anti-falsification function.
In the sheet of paper so produced, however, the thin fragments are not firmly bonded to a substrate sheet and it has been found out that fall-off of the thin fragments at the time of printing invites a serious problem. When this paper becomes spoilage or waste paper, removal of the thin fragments consisting of the polyester film is difficult, and recovery of the paper-making fibers is extremely difficult.